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Guiding Options

Guiding with Michelle (and T-ReCS) can currently only be performed on one side of the chop. The reasons for and implications of this are discussed in more detail in the Mid-IR Resources section.

Michelle has no on-instrument wavefront sensor so observations are taken using the peripheral wavefront sensor #2 (PWFS2). Please keep in mind the magnitude limits for PWFS2 stars and note that they should be at a radius of at least about 4.0 arc-minutes from the science target, as below 4.0 arc-minutes the probe may start to shadow the detector. This is the limit used by the guide star finding algorithm in the OT. In practice, guide stars as close to 3.6" can sometimes be used, but it may be necessary to change the chop angle to avoid shadowing of one of the chop positions. The outer limit for PWFS2 guide stars can be displayed on the OT position editor and is discussed here. It is always a good idea to use the OT position editor to visualise the probe arm positions with respect to the detector and the edge of the PWFS2 patrol field.